This invention relates to the commercially feasible preparation of solvent- and/or polymer-dispersible powders and, more particularly, to nano-sized powders of this description and polymers containing same. Since the invention is particularly directed to nano-sized silica powders, e.g., silica powders having a mean particle size from the order of about 2.0 nm to about 1000 nm, for purposes of illustration the following detailed description will be restricted thereto.
Initially, it is important to understand that, in the description of this invention xe2x80x9cdispersiblexe2x80x9d in solvents and/or polymers means that the dispersed particles are not agglomerated and will possess substantially the same mean particle size as their ultimate particle size. In other words, the dispersed powders of this invention will uncharacteristically maintain their particle size distribution substantially free from xe2x80x9cclumpingxe2x80x9d or agglomeration when employed in their end-use by incorporation in a polymer.
As technology responds to commercial needs, research finds ever-increasing uses of nano-sized particles as additives to polymeric films for a whole host of functions, namely as antiblocking additives, for controlling the coefficient of friction of the film, creation of a nano-composite structure possessing improved physical characteristics, including improved tensile and flexural strength, improved tensile modulus, improved barrier properties against transmission of water vapor, oxygen and/or carbon dioxide, enhanced abrasion resistance, and control of crystallization and nucleation in polymers.
In many of these contemplated uses, it is desired that the film containing the powder additive remain xe2x80x9ceffectively transparentxe2x80x9d, i.e., a film that is transparent to the eye if not entirely transparent to optical instruments for measuring transparency.
Typical powders made of nano-sized agglomerates are extremely difficult to disperse satisfactorily and do not normally disperse to the ultimate particle size of the powder.
This can, in turn, lead to loss of transparency, haze and/or poor physical properties. Silica particles in particular firm very strong bonds which are exceedingly difficult to break, when dried from water.
Iler and others have prepared silica organosols from which substantially dispersible silica powder reportedly can be made. However, the use of solvents and the preparation of powders by solvent drying is very costly and presents many cost and safety issues.
These characteristics frame the task of this invention, namely to provide powders, namely nano-sized powders which are free from agglomeration and will on dispersion retain their initial particle size distribution as colloidal sols. As mentioned above, This problem is critical, for example, when, as alluded to above, the polymer containing the dispersed powder for any of the functions heretofore mentioned is intended to provide an effectively transparent film.
In accordance with this invention the task is solved in an elegant manner by modifying the polar or hydrophilic surface of the silica or other powder prior to dispersion to make the particle surface non-polar or organophilic.
For example, a negatively charged silica hydrosol is coated with a hydrous oxide providing a surface capable of reacting with an organic acid. Stability of the aqueous sol is preserved by reversal of the charge from negative to positive. The modified aqueous sol is then coated with an organic acid by treatment with an amphiphilic surfactant and the aforementioned organic acid. Upon drying, a powder is formed which is dispersible without agglomeration in non-polar solvents or into a polymer mix.
It will however be appreciated that if the powder particles are already positively charged, such as being colloidal or capable of reacting with an organic acid, the alumina first step can of course then be eliminated.
The thus coated powder particles may then be dispersed in a non-polar solvent for the contemplated polymers or dispersed directly in the polymer mix without danger of agglomeration and/or that the particle size would change from its initial nano size prior to dispersion.
As previously discussed, the present invention is directed to nano-sized powders, i.e., powders having a mean particle size less than one micron, particularly silica powders which are to be dispersed in a solvent or in a polymer for its contemplated uses. The problem which is confronted is that on drying these very tiny particles will tend to agglomerate and to lose their initial nano identity which, in turn, will make them unsuitable for their intended usage.
As also previously stated, this problem which defines the task of the present invention is solved by coating the surface of the particles prior to dispersion with a coating system consisting of:
(1) a hydrous oxide coating capable of reacting with an organic acid; and
(2) an overlayer of an amphiphilic surfactant and an organic acid or organic acid derivative.
A particularly useful procedure for providing the initial positively charged inorganic coating is that described and claimed in U.S. Pat. No. 3,252,917 issued May 24, 1966 to Mindick et al. entitled: METHOD OF PRODUCING ALUMINA-COATED SILICA IN SOL FORM.
As stated in Col. 2 of the patent:
xe2x80x9cIn its broadest aspects the invention comprises the discovery of a method of reacting certain acid, substantially salt-free silica sols with basic aluminum halides, allowing the reactants to remain in contact for sufficient time to coat the desired amount of alumina upon the silica particles, and finally contacting the coated sol product in a single step of ion exchange resin pass with a water insoluble anion exchange resin which has as its exchangeable anion, an anion of a weak volatile inorganic acid, to produce a substantially salt-free alumina-coated silica sol having a pH ranging from 4.5 to 6.5.xe2x80x9d
In Col. 3 under the heading xe2x80x9cSTARTING REACTANTSxe2x80x9d, it is first stated that the source of alumina is a starting material which has been defined as a basic aluminum halide which has been partly neutralized with a base or partially hydrolyzed, a preferred source of basic aluminum halide being the aluminum chlorhydroxy complex recited in U.S. Pat. Nos. 2,571,030 and 2,875,163.
Applicant considers this patent disclosure, including the patents recited therein, which disclosure is incorporated by reference herein, to be the preferred source of materials in the practice of the present invention. Specifically, as seen in the following illustrative examples, Applicant has chosen to select aluminum chlorohydrol as his preferred material for use in the practice of the present invention. However, it is to be expressly understood that the selection is a matter of function and any reagent which can modify the surface of the powder to change it from a surface of silanol groups to a surface reactive with the hydroxyl moiety of an organic acid without adversely affecting the nano-size distribution will be appropriate.
Other useful materials will be readily apparent to a person skilled in the art in the light of the instant disclosure and the objectives sought to be obtained by the present invention.
As mentioned above, the surfactant to be employed in this invention is an amphiphilic surfactant, i.e., a surfactant containing both polar water-soluble and hydrophobic water-insoluble substituents.
Since amphiphilic surfactants are per se well known in the art, the selection of the particular surfactant to be employed will be a matter of individual choice within the expected knowledge and judgment of the skilled worker. In any case, examples of particularly useful amphiphyles for the practice of the present invention include sodium dodecylsulfonate, sodium stearate and sodium dodecylsulfate.
Suitable illustrative organic acids contemplated for use in the practice of this invention include the saturated and mono-unsaturated higher fatty acids and their derivatives, e.g., oleic acid, stearic acid and palmitic acid, partially esterified phosphoric acid such as 03D (Albright and Wilson) and di-2-ethylhexyl phosphoric acid, etc.
For purposes of illustration only, the hydrous alumina-coated sol will have an alumina content equivalent to one to three atoms of aluminum per surface silicon atom in the sol. For example, a 50 nanometer sol will be ideally coated with hydrous alumina equivalent to 7.2 parts per hundred (pph) of silica. A 20 nanometer silica sol will have 18 pph alumina and a 100 nm silica sol 3.2 pph. The thus coated sol is deionized with an ion exchange resin that has been regenerated with sodium bicarbonate, e.g., IRA 67, 68, 400 and 904 from Rhom and Haas. The deionized alumina-coated sol at 20% concentration will have a specific conductivity of between 200 and 2000 micro Siemens, preferably 300-700 micro Siemens. The amphiphilic surfactant is present at one to twenty pph of the positively charged alumina coated silica sol, preferably between four and ten pph. The amount of organic acid is between one and twenty pph of the silica sol, preferably five to fifteen pph.
The following examples show by way of illustration and not by way of limitation the practice of this invention.